1. Field of the Invention
The present invention relates to flat display screens, and more particularly to so-called cathodoluminescence screens, the anode of which carries luminescent elements separated from one another by insulating areas, and likely to be excited by electron bombarding. This electron bombarding requires the biasing of the luminescent elements and can come from microtips, from layers with a low extraction potential or from a thermo-ionic source.
2. Discussion of the Related Art
To simplify the present description, only microtip screens will be considered hereafter, but it should be noted that the present invention relates generally to the various above-mentioned types of screens and the like.
FIG. 1 shows the structure of a flat color microtip display screen.
Such a microtip screen is essentially comprised of a cathode 1 with microtips 2 and of a grid 3 provided with holes 4 corresponding to the locations of microtips 2. Cathode 1 is placed facing a cathodoluminescent anode 5, a glass substrate 6 of which constitutes the screen surface.
The operating principle and a specific embodiment of a microtip screen are described, in particular, in U.S. Pat. No. 4,949,116 of the Commissariat a l'Energie Atomique.
Cathode 1 is organized in columns and is comprised, on a glass substrate 10, of cathode conductors organized in meshes from a conductive layer. The microtips 2 are implemented on a resistive layer 11 deposited on the cathode conductors and are arranged within the meshes defined by the cathode conductors. FIG. 1 partially shows the inside of a mesh and the cathode conductors do not appear on the drawing. Cathode 1 is associated with grid 3 organized in lines. The intersection of a line of grid 3 and of a column of cathode 1 defines a pixel.
This device uses the electric field which is created between cathode 1 and grid 3 to extract electrons from microtips 2. These electrons are then attracted by phosphor elements 7 of anode 5 if the latter are adequately biased. In the case of a color screen, anode 5 is provided with alternate bands of phosphor elements 7r, 7g, 7b, each corresponding to a color (Red, Green, Blue). The bands are parallel to the columns of the cathode and are separated from one another by an insulator 8, generally silicon oxide (SiO.sub.2). The phosphor elements 7 are deposited on electrodes 9, comprised of corresponding bands of a transparent conductive layer such as indium and tin oxide (ITO). The sets of red, green, blue bands are alternately biased with respect to cathode 1, so that electrons extracted from the microtips 2 of a pixel of the cathode/grid are alternately directed towards the phosphor elements 7 facing each of the colors.
The control for selecting the phosphor 7 (phosphor 7g in FIG. 1) which is to be bombarded by the electrons from the microtips of cathode 1 imposes to control, selectively, the biasing of phosphor elements 7 of anode 5, color per color.
Generally, the rows of grid 3 are sequentially biased at a potential of around 80 volts, while the bands of phosphor elements (for example 7g in FIG. 1) to be excited are biased under a voltage of around 400 volts via the ITO band on which the phosphor elements are deposited. The ITO bands, carrying the other bands of phosphor elements (for example, 7r and 7b in FIG. 1), are at a low or zero potential. The columns of cathode 1 are brought to respective potentials between a maximum emission potential and a no emission potential (for example, respectively 0 and 30 volts). The brightness of a color component of each of the pixels in a line is thus determined.
The choice of the values of the biasing potentials is linked with the features of the phosphor elements and of microtips 2. Conventionally, below a voltage difference of 50 volts between the cathode and the grid, there is no electronic emission, and the maximum emission used corresponds to a voltage difference of 80 volts.
A space 12 between substrates 6 and 10 is generally defined by means of spacers (not shown) regularly distributed on the entire surface of the screen between grid 3 and anode 5. Substrates 6 and 10 are assembled together by means of a peripheral sealing, for example, by means of a cord of fusible glass constituting, once hardened, a rigid peripheral joint.
In the case of a color screen, the tracks for connecting bands 9 by sets of bands carrying phosphor elements of a same color require the forming, on substrate 6, of a piling of insulating and conductive layers, since three sets of alternate bands have to be interconnected.
In the case of a monochrome screen, the anode of which is comprised of a plane of phosphor elements of a same color, only one connection track is needed and this track can be directly deposited on substrate 6.
A disadvantage of conventional screens is that they have a low lifetime, that is, after a relatively short operating time (of around a hundred hours), destructive phenomena due to the forming of sparks at the screen circumference occur.
The origin of this phenomenon is not well understood. It was generally thought to be due to the small space between electrodes (of around 0.2 mm) with respect to the high voltage difference between the anode and the cathode. In order to overcome, among others, this phenomenon, it had been provided to increase the distance between electrodes for a given anode/cathode voltage. However, this solution results in the occurrence of other problems (spacers, focusing . . . ) and only delays the occurrence of destructive phenomena at the screen circumference.